Even experts will sometimes advise putting a layer of gravel in the bottom of a plant pot: "for drainage." What you get in reality is a smaller pot. Think about it: if a soil is saturated such that it can hold no more water, it doesn't matter whether the bottom of the medium is a bunch of rocks or a single hole, it's gonna blow. If the soil is less than fully saturated, the water molecules will stick around in the nice spongy soil, just as a moist sponge will not leak all over your kitchen counter. The gravel does nothing.
OK, I get it, lesson learned, big deal and so what?
For one, a certain percentage of healthy plants bought from the nursery will die from drought when planted out, even if "watered" regularly. To understand this, consider that the texture* of the soil of a root ball of a balled-and-burlapped plant is invariably different than the texture of the soil into which said tree is being planted.
If the soil of the root-ball is clay, and the soil into which is being planted is sand or gravel, and if you are not mindful when watering, the water will just rush through the sand and gravel, pretty much ignoring the tightly-wound ball of clay
Reverse situation: soil of b-and-b plant is sand and the soil of its new home is clay, the water will either drain right through and be absorbed by the fine clay particles. Or worse, the plant will sit in a clay-lined pool. Even given optimum moisture, the clay lining is tough for the roots of the sandy-balled plant to penetrate, and it's much easier for the roots to swirl around in the loose sand.
So that's why you should try and match the two soil types by amending the parent soil.
But now the payoff. . . what plant on earth is adapted to germinating in one type of soil and then discovering that three feet away, in all directions, is a soil of an entirely different texture? I will submit: none. Or almost none. Because that situation does not exist in nature, and plants have never been subjected to that unlikely do-or-die evolutionary pressure. Crossing that abrupt texture barrier is just hard to do.
*texture refers to the size of the soil particles
This site is for people who like plants -- growers, enthusiasts, aesthetes, novices and professionals, those who appreciate wild things and those who appreciate the cultivated. I garden in Chelsea, and I've been visiting people's yards for 20+ years in the course of my work. My goal is to make this blog a community project, so if you share my interests, please consider becoming a participant and contributing content -- Guerin. Info: email@example.com